Crosstalk prevention in interphone systems and the like

ABSTRACT

In an interphone unit and the like including a transmitting microphone and a receiver, there are provided means for rectifying one part of the input signal into a d.c. signal and switching means connected in series with the receiver, the switching means being operated depending on the magnitude of the output d.c. signal from the rectifying means, whereby weak crosstalk signals are prevented from passing through the receiver and only communication signals are passed therethrough.

United States Patent [191 Ichikawa [11] 3,919,490 [451 Nov. 11, 1975 1 CROSSTALK PREVENTION IN INTERPHONE SYSTEMS AND THE LIKE [75] Inventor: Shigemasa lchikawa, Nagoya, Japan [73] Assignee: Aiphone Kabushiki Kaisha, Nagoya,

Japan [22] Filed: Oct. 23, 1973 [21] Appl. No.: 408,589

[30] Foreign Application Priority Data Mar. 30. 1973 Japan 48-37315 [52] 1.1.8. C1. 179/37; 179/1 VC; 179/18 AD [51] Int. Cl. H04M l/l9 [58] Field of Search 179/78, 81 A. 81 B, 100 L. 179/1 H, 1 HF, 1 VC, 170.8. 174

[56] References Cited UNITED STATES PATENTS 3.189.691 6/1965 Simpson... 179/81 B 3.286.031 11/1966 Geddes 179/1 VC 3.567.868 3/1971 Mukae et a1 179/37 3.610.835 10/1971 Reid l79/170.8 3.691.311 9/1972 Wilson 179/81 B 3.725.585 4/1973 Moniak et a1. 179/1 HF 3.742.153 6/1973 Matsuda et a1. 179/81 A 3,743,950 7/1973 Sellari. Jr. et a1. 179/1 VC Primary E.\'aminer1(athleen H. Claffy Assistant E.\'aminerGera1d L. Brigance Attorney. Agent. or Firm-Wenderoth. Lind & Ponack [57] ABSTRACT In an interphone unit and the like including a transmitting microphone and a receiver. there are provided means for rectifying one part of the input signal into a dc. signal and switching means connected in series with the receiver, the switching means being operated depending on the magnitude of the output d.c. signal from the rectifying means, whereby weak crosstalk signals are prevented from passing through the receiver and only communication signals are passed therethrough.

2 Claims. 3 Drawing Figures U.S. Patent Nov. 11, 1975 FIG.

FIG.2

FIG.3

CROSSTALK PREVENTION IN INTERPHONE SYSTEMS AND THE LIKE BACKGROUND OF THE INVENTION This invention relates generally to crosstalk preventing techniques and more particularly to a circuit for an interphone unit and the like which is connected with a great number of similar units for communication therebetween. I

In this type of interphone system, multiconductor cables each including a great number of conductors disposed in parallel (not in the form of twisted pairs) are used for interconnecting these unitsaOf these conductors, only one is used for a common return line of the system, and the rest of the conductors are usedfor individual communication lines.

Although the above described type of the interphone system has been widely used because of the economy in the required number of conductors, it involves a fundamental drawback of including a considerable amount of crosstalk in the communication signals.

The crosstalk in an interphone system and the like occurs because of the following reasons.

1. Stray capacitances are inevitably caused between the parallelly arranged conductors in a multiconductor cable, and a considerable part of the crosstalk is introduced through the stray capacitances.

2. Electromagnetic inductions also exist between the conductors, and some part of. the crosstalk is caused through the electromagnetic inductions.

3. The commonly provided return line always .includes some resistance and, hence, causesa voltage drop as a communication current flows through the return line. Such a voltage drop inevitably deleteriously affects the communications through other conductors related to the common return line.

Among the crosstalks due to the above described reasons, that produced by the stray capacitances, described in item 1, may be reduced if the load impedances connected to the individual communication lines are reduced in their magnitudes. However, when the load impedances are reduced, the crosstalkcaused by the resistance in the common return line, described in the item 3, .will be increased. This means that the crosstalk due to bothreasons, described in the itemsl and 3, cannot be eliminated by a simple procedure such as merely reducing the load impedances inserted in the communication circuits.

SUMMARY OF THE'lNVENTlON With the aforementioned difficulties in view, a primary object of the present invention is to provide an improved organization of the circuit for an interphone unit and the like, whereby any crosstalk caused therein can be eliminated .effectively without requiring any complicated circuitry.

Another object of the invention is to provide an improved circuit for an interphone unit and the like whereby any crosstalk can be prevented from entering into the receiver. regardless of the electric resistance normally existing in the common return line.

Still another object of the invention is to provide an improved circuit for eliminating crosstalk in an interphone system and the like, wherein the differencebetween the energy levels of the communication signal 2 and the crosstalk signal is utilized for preventing the crosstalk from entering into the receiver.

These and other objects of the invention, which will be made apparent from the following description, can be achieved by an improved circuit according to the invention for an interphone unit and the like including a transmitting microphone and a receiver, which circuit comprises means for rectifying one part of the input signal into a dc signal and switching means connected in series with the receiver, the switching means being operated depending on the magnitude of the do. output signal from the rectifying means, whereby weak crosstalk signals are prevented from passing through the receiver, and communication signals of greater magnitudes than the crosstalk signals are thereby allowed to pass through the receiver.

The nature, principle, and the utility of the present invention will be .more clearly understood from the following detailed description of the invention when read in conjunction with the accompanying drawing.

BRIEF DESCRlPTlON OF THE DRAWlNG [n the drawing:

FIG. 1 is a schematic circuit diagram showing an interphone system and the like having two communication lines and a return line;

FIG. 2 is a graphical representation showing the relation between the crosstalk voltage and the resistance of the common return line; and

FIG. 3 is a circuit diagram of an interphone unit and the like wherein a crosstalk preventing circuit is provided according to the present invention.

DETAILED DESCRIPTION As conducive to a better understanding of the present invention, the manner in which crosstalk due to the hereinbefore described reason 3 arises-will first be described with reference to FIG. 1.

' In the circuit diagram, two representatively indicated communication lines 1 and-l and-a single return line l provided commonly for these communication lines are all included in an interphone unit, and the resistances of these lines are denoted by r r and r,, respectively. Load impedances Z and Z are inserted at respective ends of the communication line 1,, andload imped-- ances Z and Z are inserted at-respective ends of the communication line 1 A transmitting signal source e such as a microphone is provided at the sending end of the communication line 1 thereby passing a communication current i,, through the same line l When it is assumed that the load impedances 2 through 2, are each equal'to Z, and the line resistances r and r are each equal to r,-that is,

the communication current i, with respect to a line transmitting voltage e which is divided into a return line current i, and a crosstalk current i can be expressed as follows.

wherein,

From the last ofthe above equations, it will be apparent that the crosstalk current i flowing through the line 1 increases significantly when the load impedance Z is reduced, while the line resistance r of the return line l is held constant.

Furthermore. crosstalk voltages e, and 6 appearing across the load impedances Z and Z respectively, can be expressed as follows.

The relation between the crosstalk voltage 6 or e and the line resistance r, of the common return line I, is indicated in FIG. 2 under the assumption that the load impedance Z and the line transmitting voltage e,, are held constant while the line resistances r ,v and r vary in proportion to each other.

From this relationship, it will be apparent that the crosstalk in the interphone system and the like cannot be eliminated by a simple procedure such as merely reducing the load impedances inserted in the communication lines.

The crosstalk preventing circuit according to the present invention will now be described with reference to FIG. 3 indicating a typical example of an interphone unit. A communicating terminal 1 and power source terminals 2 and 3 are provided in the interphone unit, and between the power source terminals 2 and 3, there are connected in series a transmitting microphone T and a biasing resistor R for the microphone T. The output of the microphone T is coupled to the communication terminal 1 through a required number of coupling capacitors such as C, and C The receiver R in this unit is connected through the coupling capacitor C between the communication terminal 1 and one terminal 2 of the power source terminals, which is further connected to a common return line of the interphone system. i I

According to the present invention, there is provided a crosstalk preventing circuit comprising a switching transistorQ inserted between the coupling capacitor C and the receiver R. The collector of the transistor is further connected to the power source terminal 3 through a collector resistor R,.

The base of the transistor 0 is connected through a base resistor R to an amplifier-rectifier combination 4 which is in itself ofa well known organization including an amplifying transistor Q and a voltage multiplying rectifier. The collector of the transistor 0 is connected through a resistor R to the power source terminal 3, and the base thereof is connected through a biasing resistor R to the collector.

The voltage-multiplying rectifier comprises rectifying diodes D and D and a smoothing capacitor C and the input side thereof is connected to the collector of the amplifying transistor Q through a coupling capacitor C,, while the output side thereof is coupled through the base resistor R to the base of the switching transistor Q The crosstalk preventing circuit according to the present invention operates as follows. When a weak crosstalk signal is applied across the terminals 1 and 2,

the signal is passed through the capacitors C, and C to the amplifying transistor Q of the amplifier-rectifier combination 4. The signal is amplified therein and rec tified into a dc. voltage ofa value substantially equal to a multiple ofits initial value. However. since the circuit is so composed that the bias current thus applied to the switching transistor O is not sufficient to bring the switching taansistor Q into the conducting state, the entrance of the crosstalk into the receiver R is completely prevented.

In contrast, when a communication signal of a sufficiently great amplitude is applied across the terminals 1 and 2, the signal is amplified and rectified in the amplifier-rectifier combination 4, and a d.c. voltage of a sufficiently great amplitude is applied to the base of the switching transistor 0,. The switching transistor Q is thereby brought into the conductive state. and the communication current is passed through the circuit starting from the terminal 1 and passing through the capacitor C transistor O receiver R, to the terminal 2.

From the above description, it is apparent that, according to the present invention, crosstalk can be prevented from entering the receiver effectively even in an interphone system wherein the number of required conductors is reduced to substantially one half of the interphone system employing twisted pairs of conductors.

Although the invention has been described with respect to a telephone system. it will be apparent that the circuit according to the invention may also be applied to a similar system such as an interphone system and the like. Furthermore, the circuit may also be combined into the conventional interphone system with a simple modification'thereof.

What we claim is:

1. In an interphone system including a plurality of units, each unit having a transmitting microphone, a receiver, a common terminal and at least one power source terminal, said transmitting microphone and said receiver both being coupled to said common terminal and to one power source terminal, the improvement which comprises a circuit for preventing crosstalk comprised of a combination amplifier-rectifier means having input and output sides, the input side thereof being coupled to said common terminal, said combination means rectifying one part of an input signal received by said unit at said common terminal into a d.c. signal, and switching means connected in series with said receiver and coupled to the output side of said combination means and controlled thereby, said switching means being normally open, thereby maintaining said receiver in an inoperative state, and being switched to a closed state by a d.c. signal from said combination means of above a predetermined level, said predetermined level being set so that only communication signals, rather than crosstalk from other units, switch said switching means to the closed state, said receiver being activated when said switching means is closed, whereby weak crosstalk signals are prevented from passing through the receiver and only communication signals are passed therethrough.

2. A circuit as set forth in claim 1, wherein said switching means is a switching transistor having a base, emitter and collector, said base being coupled to the output side of said combination means. 

1. In an interphone system including a plurality of units, each unit having a transmitting microphone, a receiver, a common terminal and at least one power source terminal, said transmitting microphone and said receiver both being coupled to said common terminal and to one power source terminal, the improvement which comprises a circuit for preventing crosstalk comprised of a combination amplifier-rectifier means having input and output sides, the input side thereof being coupled to said common terminal, said combination means rectifying one part of an input signal received by said unit at said common terminal into a d.c. signal, and switching means connected in series with said receiver and coupled to the output side of said combination means and controlled thereby, said switching means being normally open, thereby maintaining said receiver in an inoperative state, and being switched to a closed state by a d.c. signal from said combination means of above a predetermined level, said predetermined level being set so that only communication signals, rather than crosstalk from other units, switch said switching means to the closed state, said receiver being activated when said switching means is closed, whereby weak crosstalk signals are prevented from passing through the receiver and only communication signals are passed therethrough.
 2. A circuit as set forth in claim 1, wherein said switching means is a switching transistor having a base, emitter and collector, said base being coupled to the output side of said combination means. 